1. Field of the Invention
The present invention relates to a sheet feeding apparatus used in an image forming apparatus such as a printer, a facsimile, a copying machine, and a multi function peripheral having functions thereof.
2. Description of the Related Art
Examples of the image forming apparatus include an electrophotographic system, an offset printing system, and an inkjet printing system. In this case, a color image forming apparatus in which the electrophotographic system is used is cited by way of example to describe the related art.
From the standpoint of configuration, the color image forming apparatus is mainly divided into a tandem type in which plural image forming portions are arranged side by side and a rotary type in which plural image forming portions are arranged in a cylindrical shape. From the standpoint of transfer method, the color image forming apparatus is mainly divided into a direct transfer method for directly transferring a toner image to a sheet from a photosensitive member and an intermediate transfer method in which the toner image is transferred to the sheet after the toner image is tentatively transferred to an intermediate transfer member.
For example, there is an intermediate transfer tandem type of image forming apparatus in which four color image forming portions are arranged side by side on an intermediate transfer belt. Recently in such pieces of electrophotographic apparatus, an image forming apparatus which focuses on a print market of a small amount of circulation by utilizing an advantage that formation of a printing plate is not required unlike a printing machine. However, in order to enter the print market of the small amount of circulation, it is necessary to achieve a high-quality image, and one of factors for the high-quality image focuses on accuracy of an image position with respect to the sheet which is of a transfer material. The demand for a thin sheet of 50 g/m2 or less is enhanced.
Considering improvement of the accuracy of the image position with respect to the sheet, the accuracy of the image position is determined by registration in a sheet conveying direction, the registration in a direction orthogonal to the sheet conveying direction, magnification, and skew feeding. The skew feeding is hardly corrected by electrical control. For example, the sheet skew feeding is detected and the image position with respect to the sheet can be corrected by producing the inclined image according to the skew feeding. However, in a color image in which three or four colors are superposed, when the image is inclined in each color, a tint is changed in each sheet due to shift of dot formation in each color. Additionally it takes along time to compute the inclined image, which causes productivity to be extremely decreased. Therefore, the skew feeding is determined by performance of sheet conveying accuracy.
The method of correcting the sheet skew feeding is mainly divided in the following methods.
(1) A pair of registration rollers is disposed on an upstream side of a transfer portion, a front end of the conveyed sheet abuts on the registration rollers under suspension, and the sheet is pushed into the registration rollers to form a loop, thereby correcting the skew feeding. The registration rollers are restarted in synchronization with the image, which performs the sheet skew feeding correction and image matching in the sheet conveying direction.
(2) Using a retractable shutter instead of the registration rollers, the front end of the conveyed sheet abuts on the shutter to correct the skew feeding. Then, the sheet position is detected the image is formed while aligned with the sheet position, which performs the sheet skew feeding correction and the image matching in the sheet conveying direction.
(3) The sheet is conveyed by a skew roller while abutting on a regulating plate provided along the sheet conveying direction, the sheet skew feeding is corrected, the front end of the sheet is detected, and the image position alignment in the sheet conveying direction is performed by controlling a sheet speed (see Japanese Patent Application Laid-Open No. 11-189355).
(4) Means for detecting the skew feeding and two rollers independently driven in the direction orthogonal to the sheet conveying direction are provided, and a sheet conveying speed is changed to rotate the sheet according to an amount of sheet skew feeding, thereby correcting the skew feeding (see Japanese Patent Application Laid-Open No. 5-201587).
However, in the methods (1) to (3), because the skew feeding is corrected by utilizing stiffness (elasticity) of the sheet as the transfer material, a sufficient effect is not exerted for the thin sheet having low stiffness. On the other hand, in the method (4), because the skew feeding can be corrected without utilizing the sheet stiffness, the effect is exerted for the thin sheet having low stiffness.
However, because the correction is predictive control after the sheet skew feeding is detected in the method (4), the skew feeding detection error has a direct affect on the correction result. Therefore, the skew feeding may not completely be corrected.
It is necessary to detect the amount of skew feeding during the conveyance. One of the factors for the generation of the skew feeding detection error is a detection error due to a decrease in detection accuracy caused by an influence of speed of the detection device. Guide conveying resistance easily has an influence on the thin sheet, and an attitude of the sheet is not stabilized during the conveyance, which causes the unstable detection result of the skew feeding.
In view of the foregoing, an object of the invention is to provide a sheet feeding apparatus which can accurately correct the skew feeding to the sheet having low stiffness, and an image forming apparatus in which the sheet feeding apparatus is used.